2006 US Congressional Election

House of Representatives

Many of us are familiar with the traditional election-night maps in which
the fifty states of US are colored to indicate which political party won
the largest number of votes in the state. Such maps are a helpful visual
aid for representing the results of, for example, presidential elections,
where the country votes state-by-state for the hopeful candidates. 2006
was not a presidential election year, but the country did vote in 2006 to
elect a new House of Representatives. The appropriate map to represent the
results of this election is a slightly less familiar one, the map of
congressional districts. Here is what that map looks like for the 2006
election:

Click on the map to see a larger version

In this map each congressional district is shown either in red or blue to
indicate whether the district was won by the Republican or Democratic
candidate respectively. (By an odd twist of recent electoral history, the
accepted color scheme for the left- and right-leaning parties in the States
is the opposite of what it is in most other countries, with red indicating
right-wing not left.)

While maps like this are certainly interesting and useful, they are in some
respects difficult to read. In particular, since the map above contains
much more red than blue it gives the superficial impression that the
Republican party dominated the election, when in fact the reverse is true
– the Democrats won a significant majority of districts and with them
the control of the House of Representatives. The source of the problem, as
many people have pointed out, this is that the map fails to take account of
the widely varying population densities of the various districts. This map
shows the population density of the country, and it's easy to see that the
population is far denser in some places (the light areas) than others (the
dark areas):

Click on the map to see a larger version

Because densely populated areas of the country tend to vote Democratic, the
"blue" districts occupy smaller area on average, but they are nonetheless
large in terms of numbers of people, which is what matters in an election.
We can correct for this by making use of a cartogram, a map in which
the sizes of districts are rescaled according to their population. That
is, districts are drawn with a size proportional not to their sheer
topographic acreage – which has little to do with politics –
but to the number of their inhabitants, districts with more people
appearing larger than districts with fewer, regardless of their actual area
on the ground.

Here are the 2006 House election results on a population cartogram of this
type:

Click on the map to see a larger version

This map makes it clear that the numbers of people in Republican and
Democratic areas are in fact not so far apart, and where the first map
above makes it seem as though there is far more red than blue this one
allows one to see how the votes fall in terms of actual population.

The actual number of Representatives in the House from each party however
does not depend on how many people there are in each congressional
district: by definition there is one representative per district. If one
wants to be able to see at a glance which party has a majority in the
House, therefore, a slightly different cartogram is appropriate, one in
which we rescale the size of each congressional district to be the
same. Here is such a cartogram for the 2006 election:

Click on the map to see a larger version

In this map each district is the same size, so the total area of red is
simply proportional to the total number of Republican Representatives and
similarly for the total area of blue. In fact, this map is not so
different from the previous one: the whole point of congressional districts
is to try and divide the country up into parcels of roughly equal
population, so if we have done it right the population cartogram should
give equal area to each congressional district.

Senate

Can we do the same thing for the Senate? We can, but it's a little more
difficult. Senators are elected by popular vote in their state and hence
one could in principle make a map in which states were colored according to
the results of the corresponding vote. However, under the US electoral
system voters don't vote for the entire Senate in each election, as they do
for the House of Representatives. Instead, senators have six year terms
and a only third of Senate seats come up for election every two years.
Thus in 2006 there were elections for 33 out of the 100 seats in the Senate
and hence only 33 out of the 50 states had senate votes. If we want to
make a map, therefore, we have to decide what to do with the remaining
states.

There are several ways one could go with this, but as a representation of
the state of the country I find it useful to fill in all the states, using
not just the party affiliations of the senators elected in 2006, but
putting in all the senators. This however introduces its own problems
because there are two senators per state and some states are represented by
one Republican and one Democrat. (There are 15 such states at present.)
To represent this state of affairs, I've colored states blue, red, or
purple, thus:

Click on the map to see a larger version

As before, the actual areas of the different colors doesn't reflect the
distribution of the population but we can correct this by redrawing the map
as a cartogram. The result looks like this:

Click on the map to see a larger version

This map appears to have more blue than red, which is not an illusion: it
reflects the fact that although there are only slightly more senators in
the Democratic caucus than the Republican one (51 versus 49), the number of
people in blue states significantly outnumbers the number of people
in red ones, because the blue ones have higher populations on average.
Using the latest figures from the US Census, for instance, we find that the
total population of blue states is 167.9 million people, while the total
population of red states is 125.2 million. (In this calculation, I split
the populations of the purple states 50:50.)

Notes:

Thanks: Thanks to K. Pizzini for suggesting the cartogram with equal
areas for the districts.

Alaska and Hawaii: Alaska and Hawaii are missing from the maps
because I don't have gridded population data for these states. If anyone
knows where such data are available (for free) at the 0.5 arc-minute
resolution level or better, please drop me a line.

Data: If you are interested in the raw data for the election
winners, I've put Excel spreadsheet files here: House, Senate. Some of the
election winners were provisional at the time I made these maps. The
results may have changed slightly by the time you read this.

Technical details: The cartograms on this page were made using the
cart software
package, which implements the Fourier-transform-based cartogram
technique of Gastner and Newman. The cartograms were calculated and
rendered on a 4608×3072 grid and the calculations took about half an
hour on a standard desktop computer running the Fedora Linux operating system. The
basic images were created using a specially written rendering program and
some artistic refinements, such as the drop-shadows, were added using Gimp, a free image manipulation program.
Population data came from the 2000 US
Census, as did data for the boundaries of the states and congressional
districts.